1. Field of the Invention
This invention relates to a read circuit for a digital magnetic tape transport and more particularly to a dual density read circuit for reading data in either an 800 bpi NRZI format or a 1600 bpi phase encoded format.
2. Description of the Prior Art
Digital magnetic tape transports receive and store digital information in any one or more of a variety of formats and densities. A frequently offered tape transport arrangement is one in which information may be recorded or read in either an 800 bpi NRZI standard format or a 1600 bpi phase encoded format. Separate read circuitry is conventionally used for reading each recording format.
As information recorded in an 800 bpi NRZI format is being read from tape a pulse is generated on a read head signal for each occurrence of a one and no pulse is generated for the occurrence of a zero. A one pulse may be of either polarity and the polarity alternates for each occurrence of a one. Data bits have a nominal position spacing of 800 bits per inch on the magnetic tape with a flux transition occurring at each one bit position and no flux transition occurring at each zero bit position.
The 1600 bpi phase encoded recording format is considerably different from the 800 bpi NRZI format. The tape is divided into successive data bit intervals with a nominal density of 1600 data bit intervals per inch. A flux transition occurs at the boundary between each adjacent pair of data bit intervals with the polarity of the transition indicating a one or a zero. A phase transition may occur at the middle of a data bit interval where necessary to enable a subsequent data flux transition at the end of the interval to have a desired polarity.